## Background
When a divider cell is deleted from an index interior page, the
following algorithm is used:
1. Find predecessor: Move to largest key in left subtree of the current
page. This is always a leaf page.
2. Create replacement: Convert this predecessor leaf cell to interior
cell format, using original cell's left child page pointer
3. Replace: Drop original cell from parent page, insert replacement at
same position
4. Cleanup: Delete the taken predecessor cell from the leaf page
<img width="845" height="266" alt="Screenshot 2025-07-16 at 10 39 18"
src="https://github.com/user-
attachments/assets/30517da4-a4dc-471e-a8f5-c27ba0979c86" />
## The faulty code leading to the bug
The error in our logic was that we always expected to only traverse down
one level of the btree:
```rust
let parent_page = self.stack.parent_page().unwrap();
let leaf_page = self.stack.top();
```
This meant that when the deletion happened on say, level 1, and the
replacement cell was taken from level 3, we actually inserted the
replacement cell into level 2 instead of level 1.
## Manifestation of the bug in issue 2106
In #2106, this manifested as the following chain of pages, going from
parent to children:
3 -> 111 -> 119
- Cell to be deleted was on page 3 (whose left pointer is 111)
- Going to the largest key in the left subtree meant traversing from 3
to 111 and then from 111 to 119
- a replacement cell was taken from 119
- incorrectly inserted into 111
- and its left child pointer also set as 111!
- now whenever page 111 wanted to go to its left child page, it would
just traverse back to itself, eventually causing a crash because we have
a hard limit of the number of pages on the page stack.
## The fix
The fix is quite trivial: store the page we are on before we start
traversing down.
Closes#2106Closes#2108
When a divider cell is deleted from an index interior page, the following
algorithm is used:
1. Find predecessor: Move to largest key in left subtree (self.prev())
2. Create replacement: Convert predecessor leaf cell to interior cell format, using original cell's left child pointer
3. Replace: Drop original cell from parent page, insert replacement at same position
4. Cleanup: Delete predecessor from leaf page
The error in our logic was that we always expected to only traverse down
one level of the btree:
```rust
let parent_page = self.stack.parent_page().unwrap();
let leaf_page = self.stack.top();
```
This meant that when the deletion happened on say, level 1, and the replacement
cell was taken from level 3, we actually inserted the replacement cell into
level 2 instead of level 1.
In #2106, this manifested as the following chain of pages, going from parent to children:
3 -> 111 -> 119
Cell was deleted from page 3 (whose left pointer is 111), and a replacement cell was taken
from 119, incorrectly inserted into 111, and its left child pointer also set as 111!
The fix is quite trivial: store the page we are on before we start traversing down.
Closes#2106
## Background
PR #2065 fixed a bug with table btree seeks concerning boundaries of
leaf pages.
The issue was that if we were e.g. looking for the first key greater
than (GT) 100, we always assumed the key would either be found on the
left child page of a given divider (e.g. divider 102) or not at all,
which is incorrect. #2065 has more discussion and documentation about
this, so read that one for more context.
## This PR
We already had similar handling for index btrees as #2065 introduced for
table btrees, but it was baked into the `BTreeCursor` struct's seek
handling itself, whereas #2065 handled this on the VDBE side.
This PR unifies this handling for both table and index btrees by always
doing the additional cursor advancement in the VDBE.
Unfortunately, unlike table btrees, index btrees may also need to do an
additional advance when they are looking for an exact match. This
resulted in a bigger refactor than anticipated, since there are quite a
few VDBE instructions that may perform a seek, e.g.: `IdxInsert`,
`IdxDelete`, `Found`, `NotFound`, `NoConflict`. All of these can
potentially end up in a similar situation where the cursor needs one
more advance after the initial seek, and they were currently calling
`cursor.seek()` directly and expecting the `BTreeCursor` to handle the
auto-advance fallback internally.
For this reason, I have 1. removed the "TryAdvance"-ish logic from the
index btree internals and 2. extracted a common VDBE helper `fn
seek_internal()` - heavily based on the existing `op_seek_internal()`,
but decoupled from instructions and the program counter - which all the
interested VDBE instructions will call to delegate their seek logic.
Closes#2083
Reviewed-by: Nikita Sivukhin (@sivukhin)
Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Closes#2084
PR #2065 fixed a bug with table btree seeks concerning boundaries
of leaf pages.
The issue was that if we were e.g. looking for the first key greater than
(GT) 100, we always assumed the key would either be found on the left child
page of a given divider (e.g. divider 102), which is incorrect. #2065 has more
discussion and documentation about this, so read that one for more context.
Anyway:
We already had similar handling for index btrees, but it was baked into
the `BTreeCursor` struct's seek handling itself, whereas #2065 handled this
on the VDBE side.
This PR unifies this handling for both table and index btrees by always doing
the additional cursor advancement in the VDBE.
Unfortunately, since indexes may also need to do an additional advance when they
are looking for an exact match, this resulted in a bigger refactor than anticipated,
since there are quite a few VDBE instructions that may perform a seek, e.g.:
`IdxInsert`, `IdxDelete`, `Found`, `NotFound`, `NoConflict`.
All of these can potentially end up in a similar situation where the cursor needs
one more advance after the initial seek.
For this reason, I have extracted a common VDBE helper `fn seek_internal()` which
all the interested VDBE instructions will call to delegate their seek logic.
- Apart from regular states Found/NotFound seek result has TryAdvance
value which tells caller to advance the cursor in necessary direction
because the leaf page which would hold the entry if it was present
actually has no matching entry (but neighbouring page can have match)
Closes#2047
the validation code was assuming that:
- if the parent has overflow cells after inserting a divider cell
- the exact divider we are validating MUST be in those overflow cells
However, this is not necessarily the case. Imagine:
- First divider gets inserted at index `n`. It is too large to fit, so
it gets pushed to `parent.overflow_cells()`. Parent usable space does
not decrease.
- Second divider gets inserted at index `n+1`. It is smaller, so it
still fits in usable space.
Hence:
Provide information to the validation function about whether the
inserted cell overflowed, and use that to find the left pointer and
assert accordingly.
Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Closes#2050
the validation code was assuming that:
- if the parent has overflow cells after a inserting divider cell
- the exact divider we are validating MUST be in those overflow cells
However, this is not necessarily the case. Imagine:
- First divider gets inserted at index `n`. It is too large to fit,
so it gets pushed to `parent.overflow_cells()`. Parent usable space
does not decrease.
- Second divider gets inserted at index `n+1`. It is smaller, so it
still fits in usable space.
Hence:
Provide information to the validation function about whether the inserted
cell overflowed, and use that to find the left pointer and assert accordingly.
We can use `right_page_id` directly to perform the validation instead of
carrying a raw pointer around which might be invalidated by the time we
do the validation.
